The invention relates to the field of optical recording and is most closely related to channel decoding signals read from optical media.
The invention relates to a method of decoding a stream of channel bits of a signal relating to a binary channel into a stream of source bits of a signal relating to a binary source.
The invention also relates to a device for decoding a stream of channel bits of a signal relating to a binary channel into a stream of source bits of a signal relating to a binary source, which device comprises decoding means conceived to decode a main channel.
The invention is applicable to record carriers with different kinds of channel codes. In a channel code source bits are encoded to channel bits according to a predetermined scheme. The information stored on these record carriers can be coded, for example, in accordance with a runlength-limited (RLL) code. A RLL code is characterized by two parameters, (d+1) and (k+1), which stipulate the minimum and maximum runlengths, respectively, that may occur in the code. The length of time usually expressed in channel bits between consecutive transitions is known as the runlength. Such a transition can for example be a transition from a pit-mark to a land-mark, e.g. in CD-DA, CD-R, or a transition from an amorph domain to a crystalline domain, e.g. in CD-RW.
The functioning of a method mentioned in the foregoing can be understood from UK Patent Application GB 2 083 322 (PHQ 80007). This document describes a device for decoding a stream of channel bits into a stream of source bits. In this case, the binary channel signal to be decoded is runlength-limited. This stream of channel bits is obtained by reading a record carrier with a focused laser beam, as is typical for optical record carriers. Using this RLL code and this reading technique results in record carriers with a reasonably high capacity. In UK Patent Application GB 2 083 322 the channel code called EFM, employed in the Compact Disc Digital Audio standard, is disclosed.
However, under the conditions of the beam spot diameter (depending on the NA of the objective lens used) and the wavelength of the present laser beam, the capacity of the record carrier cannot be increased when keeping identical detection margins.
Those skilled in the art are hereby directed to U.S. Pat. No. 5,642,113 by Schouhamer, European Publication EP 0866454, and International Publication WO 97/35304.
The above citations are hereby incorporated in whole by reference.
The invention has for an object to increase the data capacity of a record carrier under the conditions mentioned above.
In this invention, the capacity of the record carrier is increased by addition of a secondary channel on top of a main channel and by providing a reliable detection of this secondary channel. The main channel is a binary channel where pits and non-pits (lands) are related to two possible signal levels (below and above a threshold level).
The method in accordance with the invention is characterized in that the binary channel comprises a main channel and a secondary channel, the secondary channel being embedded in the main channel, and that a stream of corrected main channel bits is used for correcting errors in the stream of secondary channel bits.
The invention is based on the recognition that errors in the stream of bits of the binary channel relating to the secondary channel are correlated with the marks of the main channel. In order to correct these errors, the stream of error-corrected bits of the binary source relating to the main channel must be re-encoded into the stream of error-corrected bits of the binary channel relating to the main channel.
When establishing this interaction between error-correction of the main channel and error correction of the secondary channel, a reliable secondary channel is created. It should be noted that the secondary channel exists by virtue of the main channel due to its hierarchical structure.
By embedding a secondary channel extra capacity can be generated on top of the capacity of the main channel. When reading out a record carrier comprising a secondary channel a traditional player can only see the information stored in the main channel, while enhanced player, equipped with means for reading out and decoding the secondary channel, can also see the information stored in the secondary channel.
Another method in accordance with the invention is characterized in the stream of corrected main channel bits is reconstructed from a stream of corrected source bits.
Another method in accordance with the invention is characterized in that the secondary channel is embedded in the main channel via multi-level coding.
Multi-level coding can be achieved in different ways. With multi-level coding is meant the coding which uses the different levels of the read-out signal, obtained when reading out a record carrier, to decode the data. These different levels of the read-out signal can for example be accomplished by changing the geometry of a pit or mark to be read out from the record carrier. This change in geometry can be of different kinds, variation of the width, depth, the number of width or depth variations etc. A physical parameter of the secondary channel can be used for multi-level coding, e.g. a so-called xe2x80x9cpeanutxe2x80x9d-structure can be made, or the depth and/or width of the pits and marks can be varied.
Another method in accordance with the invention is characterized in that the secondary channel is embedded in the main channel via merging-bit coding.
In channel coding source bits are encoded to channel bits. In some channel codes, merging bits are inserted between the channel bits in order to be able to influence some properties of a modulated channel signal comprising the channel bits. Using merging-bits, for example the DC-content of the modulated channel signal, formed by concatenating the encoded channel bits, can be controlled. In merging-bit coding part of the freedom that is offered in the choice of the merging bits is used. Some of the merging-bit patterns will be used for DC-control, the others will be used for the generation of extra capacity.
Another method in accordance with the invention, wherein the binary channel comprises merging-bits forming merging-bit patterns of a first and of a second class, is characterized in that the merging-bit patterns of the first class are used for influencing a property of the binary channel, and the merging-bit patterns of the second class are used for creating the secondary channel.
This method is based on the insight that not all merging-bit patterns are equally suited for influencing the properties of the binary channel. Therefore, a distinction is made between merging-bit patterns of a first class that are better to be used for DC-control, and merging-bit patterns of a second class that are better to be used for extra capacity in the merging-bit channel.
Another method in accordance with the invention is characterized in that the multi-level coding is only applied for a predetermined minimum runlength.
The parameter nmin, which indicates this minimal runlength for which the secondary channel is created, is chosen to be such that normal timing recovery in the main channel is not affected (it should therefore be noted that the record carrier with the multi-level coded pits and marks can still be read with a conventional decoder). For example, for DVD, a reasonable value for nmin is 6, since the equalized eye-pattern under DVD-conditions reaches saturation (i.e. maximum amplitude level for land-marks and minimum amplitude for pit-marks) already for I6-runlengths. Apart from the main channel, which carries information in the occurrence of runlengths, extra capacity is available in the amplitude level of longer runlengths (the secondary channel). The secondary channel is hierarchically dependent on the main channel, since bits relating to this secondary channel can only be accommodated at those locations in the channel bit stream, where the main channel coding uses longer runlengths. This secondary channel is realized via limited multi-level (LML) coding. The limitation consists of the choice that multi-level coding is only applied for runlengths Inmin or greater, in which nmin is a predetermined value.
Another method in accordance with the invention is characterized in that a physical parameter of a reconstructed signal corresponding to the stream of corrected main channel bits is used for correcting errors in the stream of secondary channel bits.
In order to create a reliable secondary channel, errors in the stream of secondary channel bits must be corrected. Error correction for the stream of secondary channel bits consists of two stages. The first stage concerns bit-deletion and bit-insertion errors in the secondary channel that are caused by transition-shift channel errors of the main channel. The second stage concerns the more usual type of bit-flip errors, for which a standard error-correction procedure (e.g. using Reed-Solomon error-correcting codes) can be applied. The first stage of error-correction for the secondary channel is the concern of the present invention. It is advisable to use a physical parameter of the stream of bits of the binary channel relating to the main channel to accomplish this correction. After error-correcting the main channel and re-coding the stream of corrected source bits of the main channel into a stream of channel bits for the main channel, the stream of bits of the binary channel relating to the main channel is thought to be correct. Therefore, the physical parameters of this channel are thought to be correct.
Another method in accordance with the invention is characterized in that the physical parameter is the runlength.
During the detection of the stream of bits of a binary channel signal, errors may lead to erroneous runlengths in the main channel bitstream, i.e. detected runlengths may be different from encoded runlengths. Therefore, it is first assumed that each runlength carries a potential secondary channel bit, and secondary channel detection is performed on each runlength. Note that an actual secondary channel bit is detected only if the encoded runlength In is not smaller than Inmin.
Another method in accordance with the invention is characterized in that the secondary channel is split up into a secondary pit channel and a secondary land channel.
Another method in accordance with the invention is characterized in that erasure information from the main channel is used for correcting errors in the stream of bits of the binary channel relating to the secondary channel.
By using erasure information from the main channel when correcting errors in the stream of bits of the binary channel relating to the secondary channel, the traditional error-correction of the secondary channel (referred to before as the second stage of error-correction for the secondary channel) can be improved. Erasure information is information indicating the presence of possible errors in the bitstream and is generated during the error-correction of the main channel. The number of errors which can be corrected by the second stage of error-correction for the secondary channel is increased, using this erasure information.
The device in accordance with the invention is characterized in that said decoding means are also conceived to decode a secondary channel, the secondary channel being embedded in the main channel via multi-level coding, and that said decoding means are also conceived to correct errors in the stream of bits of the binary channel relating to the secondary channel using a stream of corrected bits of the binary channel relating to the main channel.
Another device in accordance with the invention is characterized in that the device further comprises reading means for reading a record carrier.
This device also comprises reading means. When reading a record carrier, the stream of bits of a binary channel signal is obtained.